"My heart is full at the extraordinary, dedicated, incredible people, too many to name individually, who have become my friends and have tirelessly helped us over these many months," said Curtis' mother, Nancy Curtis. "Please know that we will be eternally grateful."

The United Nations said Curtis was handed over to U.N. peacekeepers in the Golan Heights, which is under Israeli government control, and was given a medical checkup.

White House national security adviser Susan Rice said Curtis was safe, no longer in Syria and expected to be reunited with his family shortly.

Americans detained abroad 23 photos

Americans detained abroad23 photos

Jason Rezaian – Jason Rezaian, The Washington Post bureau chief in Tehran; his wife Yeganeh Salehi and two freelance journalists were detained on July 22, 2014, according to the newspaper. An Iranian official confirmed to CNN at the time that the group was being held by authorities but did not say what they were charged with.

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Americans detained abroad23 photos

Saeed Abedini – A 33-year-old U.S. citizen of Iranian birth, Abedini was sentenced to eight years in prison in January 2013. He was accused of attempting to undermine the Iranian government and endangering national security by establishing home churches. He was detained in Iran on September 26, 2012, according to the American Center for Law and Justice.

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Americans detained abroad23 photos

Robert Levinson – Retired FBI agent Robert Levinson has been missing since 2007. His family says he was working as a private investigator in Iran when he disappeared, and multiple reports suggest Levinson may have been working for the CIA. His family told CNN in January that they have long known that Levinson worked for the CIA, and they said it's time for the government to lay out the facts about Levinson's case. U.S. officials have consistently denied publicly that Levinson was working for the government, but they have repeatedly insisted that finding him and bringing him home is a "top" priority. The FBI increased its reward for information on Levinson from $1 million to $5 million, it announced in March.

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Americans detained abroad23 photos

Amir Hekmati – An Iranian court threw out a 2011 death sentence forAmir Hekmati, a former U.S. Marine charged with spying. But he was secretly retried in Iran and convicted of "practical collaboration with the U.S. government," his sister told CNN on April 11. He was sentenced to 10 years in prison, she said. Hekmati was detained in August 2011 during a visit to see his grandmother. His family and the Obama administration deny accusations he was spying for the CIA.

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Americans detained abroad23 photos

Joshua Boyle and Caitlin Coleman – Mothers Linda Boyle, left, and Lyn Coleman hold photo of their married children, Joshua Boyle and Caitlin Coleman, who were kidnapped by the Taliban in late 2012. Coleman was pregnant when she was kidnapped and is believed to have had a child in captivity.

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Americans detained abroad23 photos

Mohamed Soltan – Jailed since 2013 and sentenced to life for supporting the Muslim Brotherhood in Egypt, Mohamed Soltan has been released, the U.S. Embassy in Cairo said on Saturday, May 30, 2015. Soltan's family denies he belonged to the Brotherhood. Soltan had been a dual U.S. and Egyptian citizen, but renounced his Egyptian citizenship as a condition of his release.

Jeffrey Edward Fowle – One of three Americans detained in North Korea, Fowle was released and sent home, a State Department official told CNN on Tuesday, October 21. Fowle was accused of leaving a Bible in a hotel where he was staying. North Korea announced Fowle's detention in June, saying he had violated the law by acting "contrary to the purpose of tourism." Fowle told CNN: "I've admitted my guilt to the government and signed a statement to that effect and requested forgiveness from the people and the government of the DPRK."

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Americans detained abroad23 photos

Kenneth Bae – In May 2013, a North Korean court sentenced Bae, a U.S. citizen, to 15 years of hard labor for committing "hostile acts" against the state. North Korea claimed Bae was part of a Christian plot to overthrow the regime. In a short interview with CNN on Monday, September 1, Bae said he is working eight hours a day, six days a week at a labor camp. "Right now what I can say to my friends and family is, continue to pray for me," he said. After months in detention, he and fellow American detainee Matthew Todd Miller were released in November 2014.

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Americans detained abroad23 photos

Matthew Todd Miller – Miller, the American sentenced to six years of hard labor in North Korea began serving his six-year sentence on September 25. He was one of three Americans detained in North Korea who spoke to CNN's Will Ripley in September 2014 and implored the U.S. government for help. The 24-year-old was accused of tearing up his tourist visa and seeking asylum upon entry. He and Kenneth Bae were released in November 2014.

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Americans detained abroad23 photos

Peter Theo Curtis – American journalist Peter Theo Curtis was handed over to U.N. peacekeepers on August 24 after nearly two years in captivity. He is believed to have been captured in October 2012 and held by the al-Nusra Front, a Syrian rebel group with ties to al Qaeda.

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Americans detained abroad23 photos

Alan Gross – Gross, at right with Rabbi Arthur Schneier was jailed while working as a subcontractor in Cuba in December 2009. Cuban authorities say Gross tried to set up illegal Internet connections on the island. Gross says he was just trying to help connect the Jewish community to the Internet. Former President Jimmy Carter and New Mexico Gov. Bill Richardson both traveled to Cuba on Gross' behalf. He was eventually released in December 2014.

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Americans detained abroad23 photos

Sgt. Bowe Bergdahl – This undated image provided by the U.S. Army shows Bergdahl, who had been held by insurgents in Afghanistan since 2009. The White House announced Bergdahl's release on May 31, 2014. Bergdahl was released in exchange for five senior Taliban members held by the U.S. military. In March the U.S. military charged Bergdahl with one count each of "Desertion with Intent to Shirk Important or Hazardous Duty," and "Misbehavior Before The Enemy by Endangering the Safety of a Command, Unit or Place."

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Americans detained abroad23 photos

Merrill Newman – U.S. tourist and Korean War veteran Merrill Newman arrives at the Beijing airport on December 7, 2013, after being released by North Korea. Newman was detained in October 2013 by North Korean authorities just minutes before he was to depart the country after visiting through an organized tour. His son Jeff Newman said the Palo Alto, California, man had all the proper paperwork and set up his trip through a North Korean-approved travel agency.

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Americans detained abroad23 photos

Yanira Maldonado – Mexican authorities arrested Yanira Maldonado, a U.S. citizen, right, in May 2013, for alleged drug possession. She and her husband, Gary, were traveling from Mexico back to the United States when their bus was stopped and searched. She was released a few days later and is now back in the United States.

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Americans detained abroad23 photos

Laura Ling and Euna Lee – North Korea has arrested Americans before, only to release them after a visit by a prominent dignitary. Journalists Laura Ling, center, and Euna Lee, to her right, spent 140 days in captivity after being charged with illegal entry to conduct a smear campaign. They were freed in 2009 after a trip by former President Bill Clinton.

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Americans detained abroad23 photos

Aijalon Gomes – Former President Jimmy Carter negotiated the release of Aijalon Gomes, who was detained in 2010 after crossing into North Korea illegally from China. Analysts say high-level visits give Pyongyang a propaganda boost and a way to save face when it releases a prisoner.

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Americans detained abroad23 photos

Eddie Yong Su Jun – Detained in April 2011, Junwas released by North Korea a month afterward. His alleged crime was not provided to the media. The American delegation that secured his freedom included Robert King, the U.S. special envoy for North Korean human rights issues.

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Americans detained abroad23 photos

Robert Park – Without any apparent U.S. intervention, Park was released by North Korea in 2010. The Christian missionary crossed into North Korea from China, carrying a letter asking Kim Jong Il to free political prisoners and resign. North Korea's state-run news agency said Park was released after an "admission and sincere repentance of his wrongdoings." Here, Park holds a photo of Kim and a malnourished child during a protest in Seoul.

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Americans detained abroad23 photos

Josh Fattal, Sarah Shourd and Shane Bauer – Josh Fattal, center, Sarah Shourd, left, and Shane Bauer were detained by Iran while hiking near the Iraq-Iran border in July 2009. Iran charged them with illegal entry and espionage. Shourd was released on bail for medical reasons in September 2010; she never returned to face her charges. Bauer and Fattal were convicted in August 2011, but the next month they were released on bail and had their sentences commuted.

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Americans detained abroad23 photos

Haleh Esfandiari – Esfandiari, an Iranian-American scholar, was detained at Iran's Evin Prison, spending months in solitary confinement before Iran released her on bail in August 2007. Esfandiari was visiting her ailing mother in Tehran when she was arrested and charged with harming Iran's national security.

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Americans detained abroad23 photos

Robert Becker – Sixteen Americans were among the dozens arrested in December 2011 when Egypt raided the offices of 10 nongovernmental organizations that it said received illegal foreign financing and were operating without a public license. Many of the employees posted bail and left the country after a travel ban was lifted a few months later. Robert Becker, right, chose to stay and stand trial. He spent two years in prison and has since returned to the U.S.

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Americans detained abroad23 photos

Timothy Tracy – Filmmaker Timothy Tracy was arrested in Venezuela in April 2013 on allegations of funding opponents of newly elected President Nicolas Maduro, successor to the late Hugo Chavez. Tracy went to Venezuela to make a documentary about the political division gripping the country. He was released in June 2013.

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He was on his way to Tel Aviv, according to a senior administration official.

Nancy Curtis spoke to her son briefly by phone Sunday.

"He sounded so happy and excited to be free," she said. He told her "I can't believe they let me out."

His cousin Viva Hardigg said, "We've heard that his health appears good. That was very encouraging."

The United States was not involved in negotiations for his release but was aware of private efforts to secure the release, two U.S. law enforcement officials said. It's not known whether any ransom was paid, the officials said.

Curtis' release comes just five days after ISIS militants released a video of one of its militants beheading American journalist James Foley.

"Particularly after a week marked by unspeakable tragedy, we are all relieved and grateful knowing that Theo Curtis is coming home after so much time held in the clutches of (al-Nusra Front)," U.S. Secretary of State John Kerry said in a statement.

When asked about the death of James Foley, an American journalist killed by the Islamic State in Iraq and Syria last week, Nancy Curtis began to weep. "I feel so bad for their family. Diane (Foley's mother) has become a good friend of mine, and I'm just so heartbroken for their loss."

Author and journalist

CNN obtained two videos that appear to have been recorded during the late stages of Curtis' captivity. In one, a gun is pointed at his head, and Curtis speaks rapidly, as if under duress.

He gives his name and the date and says he is a journalist from Boston.

He has also published two books: "My Life Had Stood a Loaded Gun," a memoir about teaching literature to young offenders at a correctional facility in Vermont, and "Undercover Muslim: A Journey into Yemen," which investigates Islamic extremism.

He was born in Atlanta and graduated from Middlebury College in Vermont. Curtis holds a doctorate in comparative literature from the University of Massachusetts and is fluent in French and Arabic, according to a statement from his family. He also speaks German and Russian.

"Theo has a deep concern and regard for the people of Syria," his mother said, "which is why he returned during the war. He wanted to help others and to give meaning and to bear witness to their struggles.

"I am very fortunate that I do not have to tell his whole story. He eventually will be able to do so himself."

President Barack Obama has been briefed on Curtis' release and "shares in the joy and relief that we all feel now that Theo is out of Syria and safe," said White House spokesman Eric Schultz.

"But we continue to hold in our thoughts and prayers the Americans who remain in captivity in Syria -- and we will continue to use all of the tools at our disposal to see that the remaining American hostages are freed."

Former cellmate speaks

In August, CNN spoke to Curtis' former cellmate in Syria, Matthew Schrier. They were locked up together in six prisons before Schrier broke free by climbing through a window.

"I took apart the screen, pushed the sandbags aside, and I got stuck, around my waist, so I had to reach in. I unbuckled my pants, and as soon as I unbuckled my pants I shot right out," Schrier said.

Curtis wasn't as lucky. He got stuck. Schrier said he tried to get Curtis out, but he simply didn't fit, and so Schrier left, promising to get help.

"It was one of the hardest things I ever had to do," Schrier said.

"It's hard to move on, because he's still there. You know, it hasn't ended yet 100%," he said then. "I'm not going to have closure until he's home."